This application claims the priority of Japanese Patent Applications No. 2001-288182 filed on Sep. 21, 2001 and No. 2001-295270 filed on Sep. 27, 2001 which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an endoscope, and more particularly, to supply of illumination light and signal processing in an endoscope that allows observation of fine structure, enlarged by optical zooming, of blood vessels or the like of an object to be observed.
2. Description of the Related Art
An endoscope, for example, an electronic endoscope takes an image, by an image pickup device such as a CCD (Charge Coupled Device), of an object to be observed, captured via an objective optical system by applying illumination light, and displays an image of the object to be observed on a monitor or the like. Recently, a movable lens has been incorporated into the objective optical system, and the movable lens is moved back and forth by a zoom mechanism to optically enlarge the image of the object to be observed. The enlarged image is processed and displayed on the monitor or the like, and allows observation of details of a site to be observed.
By the above described endoscope, digestive organs in vivo are often observed, and it is important to observe blood vessels (blood capillaries) or other tissue near a surface of mucosa in vivo. Specifically, useful diagnosis information can be obtained from vascularization and spreading manner of the blood vessels, fine structure of the mucosa in vivo, or the like. However, there is a problem that hemoglobin in blood also exists in peripheral tissue, thus entire sites in vivo have redness to cause unclear distinction between the mucosa and the blood vessel, or other tissue.
Such unclear distinction between the mucosa and the blood vessel can be recognized by spectral reflectances mentioned below. FIG. 9A shows a spectral reflectance of normal gastric mucosa (wavelengths of 400 to 800 nm), FIG. 9B shows a spectral reflectance of human blood, FIG. 9C shows overlapped characteristic curves of the spectral reflectances of the normal gastric mucosa and the blood. As shown in FIG. 9C, a spectral reflectance curve C1 of the normal gastric mucosa and a spectral reflectance curve C2 of the blood cross near a wavelength of 600 nm. The spectral reflectance of the blood becomes significantly high over the wavelength of 600 nm.
FIG. 10 shows a depth that each color contained in illumination light reaches in a lower layer of the mucosa. The longer the wavelength of the light becomes from blue to red, or the longer the wavelength of red light becomes, more scattering occurs in the lower layer of the mucosa. The scattering of the long wavelength light prevents taking a good image of the blood vessels near the mucosa.
In the past, to avoid saturation of a CCD used as an image pickup device by infrared radiation, an infrared cut optical filter for cutting infrared radiation has been used. FIG. 11 shows an example of a spectral transmittance characteristic of a recent infrared cut filter, and this filter has a characteristic that the spectral transmittance becomes half near 660 nm and zero at 700 nm.
FIG. 12 shows spectral reflectances of the mucosa and the blood when using the above described infrared cut filter (corresponding to FIG. 9C). As shown in FIG. 12, this infrared cut filter cuts light in an infrared band according to a characteristic curve in FIG. 11. Specifically, a wavelength band from near 700 nm and higher is removed, and a long wavelength side of the spectral reflectance of the blood is also cut.
However, the infrared cut filter is simply for removing the infrared radiation, and as described above, scattering, below the mucosa, of a long wavelength component among red lights causes the image of the object to be observed to have redness, and contrast for sufficient distinction between the mucosa, the blood vessels, and other tissue cannot be obtained.
For the spectral reflectance characteristic in FIG. 12, an area S1 surrounded by curves C1 and C2 of wavelengths from 400 nm to near 600 nm shows a component contributing to the contrast between the mucosa, the blood vessels, and other tissue in the image of the object to be observed, and an area S2 surrounded by curves C1 and C2 of wavelengths near 600 nm and higher shows a component causing light scattering in the lower layer of the mucosa and reducing the contrast between the mucosa and the blood, or the like, and a size of the area S2 becomes a problem.